Decodable indicia such as bar codes and OCR decodable characters are finding increased use in an ever expanding variety of applications. For example, bar codes are being applied to paper substrate surfaces, plastic bags, glass, and directly on finished articles. Decodable indicia that can be applied deliberately to objects include a variety of formats, comprising geometrical features (e.g., one-dimensional (1D) symbols, two-dimensional (2D) symbols), and features of tonality and/or color (e.g., symbols comprising gray scales and symbols comprising colors). Decodable indicia can also occur naturally, for example in the form of biometric indicia such as fingerprints, retinal patterns, facial features and the like. Some of these natural indicia may also be applied, deliberately or inadvertently, to other surfaces, for example as fingerprints.
Often, different types of decodable indicia require imaging modules that provide different data formats. The term “imaging module” is intended in one embodiment to describe the image sensor device itself. The sensor when disposed within a housing, and including, as required, imaging optics, lenses, filters and the like, and electronic circuitry used to operate the image sensor device or used in conjunction with the image sensor device, is referred to as an optical reader. Historically, one type of decoder module has been used with imaging modules providing data having a first format (for example, 1D data), and another type of decoder module has been used with imaging modules providing data having a second format (for example, 2D data). In general, the computational power required to decode more densely encoded decodable indicia causes decoder modules suitable for such decoding to be relatively expensive as compared to decoder modules with only sufficient computational power to decode less complex decodable indicia. This relationship is generally referred to as a “price-performance trade-off.”
A number of problems in imaging different decodable indicia arise because of the circumstances of use of the decodable indicia. For example, where decodable symbols or characters have been applied to particularly reflective “shiny” surfaces (such as glass, plastic, or metallic surfaces), “specular reflection” decode failures have been observed. “Specular reflection” occurs where a light ray incident on a highly reflective (mirror) surface is reflected at an angle substantially equal to an angle of incidence measured with respect to a direction that is substantially normal to the surface. In optical readers, light sources are positioned to emit light along a path closely adjacent a centrally located imaging axis. An optical reader light is directed at a reflective target and, therefore, the illumination light tends to be reflected secularly in the direction of the reader's photodetector elements. Specular reflection can result in the captured image data failing to exhibit adequate contrast between dark and light markings of a decodable indicia. With the increased miniaturization of optical readers, light sources for illuminating a target are being positioned in closer proximity with a photodetector element of the reader, thereby rendering the modem reader more susceptible to specular reflection read failures.
The proliferation of the use of decodable markings has brought to light additional problems with presently available optical readers. It has become more common to encode more information into single decodable indicia, e.g. with use of “high density” bar codes, to affix more than one decodable indicia in need of decoding, possibly having different formats, onto an article or package, and to make bar codes wider so that they can encode more information. “High density” bar codes are best decoded with the use of a high resolution optical reader which is configured to have a short “best focus” position. Extra wide bar codes and scenes having more than one bar code are best decoded with use of readers having a longer best focus position. Commercially available optical readers cannot easily read high density extra wide decodable symbols or multiple symbols from a scene which are encoded in high density.
There is a need for an optical reader which can decode a variety of different formats of image data, so as render it impervious to decode failures resulting from specular reflection, and which is adapted to read large or multiple high density decodable symbols, possibly having a plurality of formats, formed on a target.